Forming variable depth structures with laser ablation

ABSTRACT

A method for forming a device structure is disclosed. The method of forming a device structure includes forming a variable-depth structure in a device material layer using a laser ablation. A plurality of device structures is formed in the variable-depth structure to define slanted device structures therein. The variable-depth structure and the slanted device structures are formed using an etch process.

BACKGROUND Field

Embodiments of the present disclosure generally relate to opticaldevices for augmented, virtual, and mixed reality. More specifically,embodiments described herein provide forming depth-modulated devicestructures of optical devices.

Description of the Related Art

Virtual reality is generally considered to be a computer-generatedsimulated environment in which a user has an apparent physical presence.A virtual reality experience can be generated in 3D and viewed with ahead-mounted display (HMD), such as glasses or other wearable displaydevices that have near-eye display panels as lenses to display a virtualreality environment that replaces an actual environment.

Augmented reality, however, enables an experience in which a user canstill see through the display lenses of the glasses or other HMD device,or handheld device, to view the surrounding environment, yet also seeimages of virtual objects that are generated in the display and appearas part of the environment. Augmented reality can include any type ofinput, such as audio and haptic inputs, as well as virtual images,graphics, and video that enhances or augments the environment that theuser experiences. As an emerging technology, there are many challengesand design constraints with augmented reality.

One such challenge is displaying a virtual image overlayed on an ambientenvironment. Optical devices are used to assist in overlaying images.Generated light is propagated through a waveguide until the light exitsthe waveguide and is overlayed on the ambient environment. Fabricatingoptical devices can be challenging as optical devices tend to havenon-uniform properties. Accordingly, improved methods of fabricatingoptical devices are needed in the art.

SUMMARY

The present disclosure generally relates to a method for forming adevice structure for use in a display apparatus or in otherapplications. More specifically, the disclosure relates to a variabledepth structure for use in the device structure using created usinglaser ablation. The method herein may also form a device structure thatis used as a master for nano-imprint lithography.

In one embodiment, a method of forming a device structure is provided.The method includes forming a variable-depth structure in a devicematerial layer using laser ablation. The method also includes forming ahardmask and a photoresist stack over the device material layer. Themethod further includes etching the photoresist stack. The method alsoincludes forming a plurality of device structures in the device materiallayer.

In another embodiment, a method of forming a device structure isprovided. The method includes forming a device material layer on asubstrate and forming a variable-depth structure in the device materiallayer using laser ablation. The method also includes forming a hardmaskand a photoresist stack over the device material layer. The methodfurther includes etching the photoresist stack and forming a pluralityof device structures in the device material layer.

In yet another embodiment, a method of forming a device structure isprovided. The method includes forming a device material layer on asubstrate and forming a sacrificial layer on the device material layer.The method further includes forming a variable-depth structure in thesacrificial layer using laser ablation. The method also includes forminga hardmask and a photoresist stack over the device material layer. Themethod further includes etching the photoresist stack and forming aplurality of device structures in the device material layer.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlyexemplary embodiments and are therefore not to be considered limiting ofits scope, and may admit to other equally effective embodiments.

FIG. 1 is a front view of an optical device according to an embodiment.

FIG. 2 is a flow diagram of a method for forming a device structureaccording to an embodiment.

FIGS. 3A-3H are schematic, cross-sectional views of a portion of avariable-depth structure according to an embodiment.

FIGS. 4A-4C are cross-sectional enlargements of examples of shapes of avariable-depth structure.

FIGS. 5A-5C are perspective views of examples of three dimensionalshapes of a variable-depth structure.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation.

DETAILED DESCRIPTION

Embodiments described herein relate to methods of forming a devicestructure having variable-depth slanted device structures. To accomplishthis, a method includes forming a variable-depth structure in a devicematerial layer using laser ablation. A plurality of channels is formedin the variable-depth structure to define slanted device structurestherein. The variable-depth structure is formed using laser ablation andthe slanted device structures are formed using a selective etch process.The method described herein can also be used to create a devicestructure that functions as a master for nanoimprint lithography.

FIG. 1 is a front view of an optical device 100. It is to be understoodthat the optical device 100 described below is an exemplary opticaldevice. In one embodiment, the optical device 100 is a waveguidecombiner, such as an augmented reality waveguide combiner. In anotherembodiment, the optical device 100 is a flat optical device, such as ametasurface. The optical device 100 includes a plurality of devicestructures 104. The device structures 104 may be nanostructures havingsub-micro dimensions, e.g., nano-sized dimensions, such as criticaldimensions less than 1 μm. In one embodiment, regions of the devicestructures 104 correspond to one or more gratings 102, such as thegrating areas 102 a and 102 b. In one embodiment, the optical device 100includes a first grating area 102 a and a second grating area 102 b andeach of the first grating area 102 a and 102 b each contain a pluralityof device structures 104.

The depth of the gratings 102 may vary across the grating areas 102 aand 102 b in embodiments described herein. In some embodiments, thedepth of the gratings 102 may vary smoothly over the first grating area102 a and over the second grating area 102 b. In one example embodiment,the depth may range from about 10 nm to about 400 nm across one of thegrating areas. The grating area 102 a, in an example embodiment, canrange from approximately 20 mm to approximately 50 mm on a given side.Therefore, as one example, the angle of the change in the depth of thegratings 102 may be on the order of 0.0005 degrees.

In embodiments described herein, the device structures 104 may becreated using laser ablation. Laser ablation, as used herein, is used tocreate three-dimensional microstructures in the device material, oroptionally to create a variable-depth structure in a sacrificial layeroverlaying the device material as part of a variable-depth structureprocess. Using laser ablation to create the optical structures 104allows for fewer processing operations and higher variable-depthresolution than existing methods.

FIG. 2 is a flow diagram of a method 200 for forming a portion ofoptical device 300, shown in FIGS. 3A-3H, having variable-depthstructures, which corresponds with grating area 102 a or 102 b. Atoperation 201, a device material layer 306 is disposed over a surface ofa substrate 302 as shown in FIG. 3A. The substrate 302 may be formedfrom any suitable material, provided that the substrate 302 canadequately transmit light in a desired wavelength or wavelength rangeand can serve as an adequate support for the portion of optical device300. In some embodiments, which can be combined with other embodimentsdescribed herein, the material of substrate 302, includes, but is notlimited to, one or more silicon (Si), silicon dioxide (SiO₂), orsapphire containing materials. In other embodiments, which can becombined with other embodiments described herein, the material ofsubstrate 302 includes, but is not limited to, materials having arefractive index between about 1.7 and about 2.0.

The device material layer 306 may be disposed over the surface of thesubstrate 302 by one or more (PVD), chemical vapor deposition (CVD),plasma-enhanced (PECVD), flowable CVD (FCVD), atomic layer deposition(ALD), or spin-on processes. In one embodiment, which can be combinedwith other embodiments described herein, the device material of devicematerial layer 306 is selected based on the modulated depth and slantangle of each of the plurality of device structures 104 of the portionof optical device 300 and the refractive index of the substrate 302. Insome embodiments, which can be combined with other embodiments describedherein, the device material layer 306 includes, but is not limited to,one or more silicon nitride (SiN), silicon oxycarbide (SiOC), titaniumdioxide (TiO₂), silicon dioxide (SiO₂), vanadium (IV) oxide (VOx),aluminum oxide (Al₂O₃), indium tin oxide (ITO), zinc oxide (ZnO),tantalum pentoxide (Ta₂O₅), silicon nitride (Si₃N₄), zirconium dioxide(ZrO₂), or silicon carbon-nitride (SiCN) containing materials. In someembodiments, which can be combined with other embodiments describedherein, the device material of the device material layer 306 may have arefractive index between about 1.5 and about 2.65. In other embodiments,which can be combined with other embodiments described herein, thedevice material of the device material layer 306 may have a refractiveindex between about 3.5 and about 4.0

In some embodiments, which can be combined with other embodimentsdescribed herein, an etch stop layer 304 may be optionally disposed onthe surface of the substrate 302 between the substrate 302 and thedevice material layer 306. The etch stop layer 304 may be disposed byone or more PVD, CVD, PECVD, FCVD, ALD, or spin-on processes. The etchstop layer 304 may be formed from any suitable material, for exampletitanium nitride (TiN) or tantalum nitride (TaN), among others, providedthat the etch stop layer 304 is resistant to the etching processesdescribed herein. In one embodiment, which can be combined with otherembodiments described herein, the etch stop layer 304 is anon-transparent etch stop layer that is removed after the devicestructure 104 is formed. In another embodiment, the etch stop layer 304is a transparent etch stop layer.

At operation 202, a sacrificial layer 308 is formed over the devicematerial layer 306, as shown in FIG. 3B. In one embodiment, thesacrificial layer 308 is a SiN layer, SiOx layer or photoresist layer.In one embodiment, forming the sacrificial layer 308 includes disposinga resist material over the device material layer 306 and developing theresist material utilizing a lithography process. The resist material mayinclude but is not limited to, light-sensitive polymer containingmaterials. Developing the resist material may include performing alithography process, such as photolithography, digital lithographyand/or laser ablation. In this embodiment, laser ablation is performedon the sacrificial layer 308 to create a shape of a variable-depthstructure 301 within the sacrificial layer 308 over a length L with adepth of D on the left side and a depth of D′ on the right side. Asdescribed above, any desired one-, two-, or three-dimensional shape canbe created in the sacrificial layer 308 using laser ablation. Laserablation uses variable pulse repetition of a laser beam scanned acrossan area to be ablated. One benefit of laser ablation over othervariable-depth process, such as gray-tone resist processes, is thatlaser ablation is a physical process as opposed to the chemical processusing a gray-tone resist which can have a limited shelf life. Laserablation also results in faster throughput and faster changes to thevariable-depth structure without the need for masks. Laser ablation alsoresults in increased spacial fidelity or resolution over typical etchprocesses.

In this embodiment, at operation 203, a transfer etch process is thenperformed on the variable-depth structure 301 of the sacrificial layer308 to form the variable-depth structure 301 within the device materiallayer 306. The results of operation 203 are illustrated in FIG. 3C. Inthis embodiment, the transfer etch process removes the sacrificial layer308 and etches the underlying device material layer 306 to produce thevariable-depth structure 301 within the device material layer 306.

The variable-depth structure 301 in this embodiment has a length Lbetween a first end and a second end. The first end of thevariable-depth structure 301 has a depth F and the second end has adepth F′. That is, the depth of the variable-depth structure 301 isminimal at the first end and maximum at the second end in thisembodiment. The depth from F to F′ generally is within a range of about0 nm to about 700 nm. In this embodiment, the length L is substantiallylarge compared to the depths F and F′. For example, the length L may beabout 25 mm while the depth F at the first end is about 0 nm to about 50nm and the depth F′ at the second end is about 250 nm to about 700 nm.Accordingly, the variable-depth structure 301 has a substantiallyshallow slope. In this example, the angle of the slope is less than 1degree, such as less than 0.1 degrees, like about 0.0005 degrees. Theslope of the variable-depth structure 301 is illustrated herein with anexaggerated angle for clarity.

In one embodiment, which can combined with other embodiments, where thedevice design process does not require the deposition of a sacrificiallayer 308 as described above, laser ablation may be performed directlyon the device material layer 306 to form the variable-depth structure301. Laser ablation is performed to create the shape of thevariable-depth structure 301 over the length L with a depth of F on theleft side and a depth of F′ on the right side. In one embodiment, theshape of the variable-depth structure 301 over length L is in the shapeof a wedge with varying levels of depth. The shape of the variable-depthstructure 301 determines the modulation of the depth D of devicestructure 104 across the substrate 302, as shown in FIG. 3H.

At operation 204, a hardmask 312 is disposed over the device materiallayer 306 and variable-depth structure 301. The results of operation 204are illustrated in FIG. 3D. The hardmask 312 may be disposed over thedevice material layer 306 by one or more liquid material pour casting,spin-on coating, liquid spray coating, dry powder coating, screenprinting, doctor blading, PVD, CVD, PECVD, FCVD, ALD, evaporation, orsputtering processes. In one embodiment, which can be combined withother embodiments described herein, the hardmask 312 is non-transparentand is removed after the portion of optical device 300 is formed. Inanother embodiment, the hardmask 312 is transparent. In someembodiments, which can be combined with other embodiments describedherein, the hardmask 312 includes, but is limited to, chromium (Cr),silver (Ag), Si₃N₄, SiO₂, TiN, or carbon (C) containing materials. Thehardmask 312 can be deposited so that the thickness of the hardmask 312is substantially uniform. In yet other embodiments, the hardmask 312 canbe deposited so that the thickness varies from about 30 nm and about 50nm at varying points on the device material layer 306. The hardmask 312is deposited in such a way that the slope of the hardmask 312 is similarto the slope of the variable-depth structure 301.

At operation 205, an organic planarization layer 314 is disposed overthe hardmask 312. The results of operation 205 are illustrated in FIG.3E. The organic planarization layer 314 may include a photo-sensitiveorganic polymer comprising a light-sensitive material that, when exposedto electromagnetic (EM) radiation, is chemically altered and thusconfigured to be removed using a developing solvent. The organicplanarization layer 314 may include any organic polymer and aphoto-active compound having a molecular structure that can attach tothe molecular structure of the organic polymer. In one embodiment, whichcan be combined with other embodiments described herein, the organicplanarization layer 314 may be disposed using a spin-on coating process.In another embodiment, which can be combined with other embodimentsdescribed herein, the organic planarization layer 314 may include, butis not limited to, one or more of polyacrylate resin, epoxy resin,phenol resin, polyamide resin, polyimide resin, unsaturated polyesterresin, polyphenylenether resin, polyphenylenesulfide resin, orbenzocyclobutene (BCB).

As shown in FIG. 3E, the optical planarization layer 314 varies inthickness, such that a substantially planar top surface is formed. Theoptical planarization layer 314 varies in thickness, such that the spacebetween the sloped conformal hardmask 312 and the substantially planartop surface of the optical planarization layer 314 is completely filledand has a varying thickness over the sloped wedge shaped-structure 301.

Referring to FIGS. 3E-3H, at operation 206, a patterned photoresist 316is disposed over the organic planarization layer 314. The patternedphotoresist 316 is formed by disposing a photoresist material on theorganic planarization layer 314 and developing the photoresist material.The patterned photoresist 316 defines a hardmask pattern 315, shown inFIG. 3E that corresponds to exposed segments 321 of the device materiallayer 306, as shown in FIG. 3G. The hardmask pattern 315 functions as apattern guide for formation of slanted device structures 104. Theexposed segments 321, as shown in FIG. 3G, of the device material layer306 to be etched correspond to gaps 324 between the device structures104, as shown in FIG. 3H. In one embodiment, which can be combined withother embodiments described herein, the photoresist material may bedisposed on the organic planarization layer 314 using a spin-on coatingprocess. In another embodiment, which can be combined with otherembodiments described herein, the patterned photoresist 316 may include,but is not limited to, light-sensitive polymer containing materials.Developing the photoresist material may include performing a lithographyprocess, such as photolithography and/or digital lithography.

At operation 207, organic planarization layer portions 317 of theorganic planarization layer 314 exposed by the resist pattern 315 areremoved. Removing the organic planarization layer portions 317 exposesnegative hardmask portions 319 of the hardmask pattern 315 thatcorrespond to the gaps 324 between the device structures 104. Theorganic planarization layer portions 317 may be removed by IBE, RIE,directional RIE, plasma etching, wet etching, and/or lithography. Theresults of operation 207 are shown in FIG. 3F.

At operation 208, the negative hardmask portions 319 of the hardmaskpattern 315 are etched. The results of operation 208 are shown in FIG.3F. Etching the negative hardmask portions 319 exposes the exposedsegments 321 of the device material layer 306 corresponding to thehardmask pattern 315. In one embodiment, which can be combined withother embodiments described herein, etching the negative hardmaskportions 319 may include, but is not limited to, at least one of IBE,RIE, directional RIE, or plasma etching.

At operation 209, the patterned photoresist 316 and the organicplanarization layer 314 are removed. The results of operation 208 areillustrated in FIG. 3G. Stripping the organic planarization layer 314and the patterned photoresist layer 316 yields a set of negativehardmask portions 319.

At operation 210, an etch process is performed. In one embodiment, whichcan be combined with other embodiments described herein, an angledetching process is performed. The angled etch process may include, butis not limited to, at least one of IBE, RIE, or directional RIE. The ionbeam generated by IBE may include, but is not limited to, at least oneof a ribbon beam, a spot beam, or a full substrate-size beam. Performingthe angled etch process etches the exposed segments 321 of the devicematerial layer 306 to form a plurality of device structures 104. Asshown in FIG. 3H, the angled etching process forms the plurality ofdevice structures 104 such that the device structures 104 have a slantangle ϑ relative to the surface of the substrate 302. In one embodiment,which can be combined with other embodiments described herein, the slantangle ϑ of each of the device structures 104 is substantially the same.In another embodiment, which can be combined with other embodimentsdescribed herein, the slant angle ϑ of at least one device structure ofthe plurality of device structures 104 is different.

The device structure pattern 310 provides for a depth D of the devicestructures 104 to have gradient modulated across the substrate 302. Forexample, as shown in FIG. 3H, the depth D of the device structures 104decreases in the X-direction across the substrate 302. In oneembodiment, which can be combined with other embodiments describedherein, the gradient of the depth D of the device structures 104 iscontinuous. In one embodiment, which can be combined with otherembodiments described herein, the gradient of the depth D of the devicestructures 104 is step-wise. As described above, modulating the depth Dof the device structures 104 provides for control of the in-coupling andout-coupling of light by the gratings 102 of the optical device 100.

At operation 211, an optional operation may be performed to strip thehardmask 312. A wet clean may be performed in some embodiments.

The laser ablation process described herein advantageously allows thevariable-depth structure to have a slope and/or curvature in one or moredirections. FIGS. 4A-4C illustrate other examples of shapes that can beused for the variable-depth structure. FIG. 4A illustrates avariable-depth structure 420 in a device material layer 406 of a portionof optical device 400. The variable-depth structure 420 has two planarsloped portions which extend from respective peripheral regions 420 a,420 b towards a central region 420 c. FIG. 4B illustrates avariable-depth structure 450 in a device material layer 436 of a portionof optical device 430. The variable-depth structure 450 is a curvedstructure which has a shallow depth D at peripheral regions 450 a, 450 band an increased depth at a central region 450 c. In one example, thevariable-depth structure 450 has a parabolic shape. The depth Dincreases non-linearly from the peripheral regions 450 a, 450 b to thecentral region 450 c. FIG. 4C illustrates a variable-depth structure 480in a device material layer 466 of a portion of optical device 460. Thevariable-depth structure 480 has a depth D that oscillates from a firstend 480 a to a second end 480 b which forms a pattern of cyclical depthsD for the variable-depth structure 480. The variable-depth structure 480is shown with linear, saw-tooth oscillations of the depth D. However, itis contemplated that the depth D can vary non-linearly so that thevariable-depth structure has wave-like oscillations in the depth D. Thedepth D of a variable-depth structure, such as wedge-shapes structures420, 450, 480 can change linearly or non-linearly across a length Lthereof from a first end (i.e, 420 a, 450 a, 480 a) to a second end(i.e., 420 b, 450 b, 480 b). Utilizing grayscale lithography, laserablation and the techniques described herein, variable-depth structuresof varying shapes can be patterned with a single pass instead ofmultiple operations as required by prior techniques.

In another example, the variable-depth structure has a three dimensionalshape. That is, the depth changes in multiple directions (i.e., a firstdirection X and a second direction Y) as illustrated in the examples ofFIGS. 5A-5C. FIG. 5A illustrates a variable-depth structure 520 whichhas a saddle-point shaped curvature (i.e., hyperbolic paraboloid shape).FIG. 5B illustrates a variable-depth structure 550 which has an ellipticparaboloid shape with positive curvature. FIG. 5C illustrates avariable-depth structure 580 which has an elliptic paraboloid shape withnegative curvature. The three dimensional shape of the variable-depthstructure is not limited to the examples of FIGS. 5A-5C. Other desiredshapes, for example a paraboloid in a square domain with positivecurvature or negative curvature, an ellipsoid, and linear sloped shapes,among others, are also contemplated and can be used herewith. In thesecases, the depth of the variable-depth structure changes in both the Xand Y directions. Thus, upper surfaces of the slanted device structuresare curved as defined by the shape of the curvature of thevariable-depth structure.

In summation, methods for forming a device structure havingvariable-depth slanted device structures are described herein. Themethods include forming a depth-modulated variable-depth structure in adevice material layer using laser ablation. A plurality of devicestructures are formed in the variable-depth structure to define slanteddevice structures therein. The variable-depth structure is formed usinglaser ablation, and the slanted device structures are formed using anetch process. The method described herein can also be used to create adevice structure that functions as a master for nanoimprint lithography.

What is claimed is:
 1. A method of forming a device structurecomprising: forming a device material layer on a substrate; forming avariable-depth structure in the device material layer using laserablation; forming a hardmask and a photoresist stack over the devicematerial layer, wherein the photoresist stack comprises an opticalplanarization layer and a photoresist layer, wherein the photoresistlayer is in direct contact with the optical planarization layer; etchingthe photoresist stack; etching the hardmask; and forming a plurality ofdevice structures in the device material layer by etching through thedevice material layer.
 2. The method of claim 1, wherein thevariable-depth structure changes in depth from a first end to a secondend.
 3. The method of claim 2, wherein the depth of the variable-depthstructure changes linearly from the first end to the second end.
 4. Themethod of claim 2, wherein the depth of the variable-depth structurechanges non-linearly from the first end to the second end.
 5. The methodof claim 2, wherein the depth of the variable-depth structure oscillatesfrom the first end to the second end.
 6. A method of forming a devicestructure comprising: forming a device material layer on a substrate;forming a sacrificial layer on the device material layer; forming avariable-depth structure in the sacrificial layer using laser ablation;forming a hardmask and a photoresist stack over the sacrificial layer,wherein the photoresist stack comprises an optical planarization layerand a photoresist layer, wherein the photoresist layer is in directcontact with the optical planarization layer; etching the photoresiststack; etching the hardmask; and forming a plurality of devicestructures in the device material layer by etching through the devicematerial layer.
 7. The method of claim 6, wherein the variable-depthstructure changes in depth from a first end to a second end.
 8. Themethod of claim 6, wherein the depth of the variable-depth structurechanges linearly from the first end to the second end.
 9. The method ofclaim 6, wherein the depth of the variable-depth structure changesnon-linearly from the first end to the second end.
 10. The method ofclaim 6, wherein the depth of the variable-depth structure oscillatesfrom the first end to the second end.
 11. The method of claim 10,wherein the etching of the sacrificial layer and the device materiallayer results in the transfer of the variable-depth structure into thedevice material layer.
 12. The method of claim 6, further comprising;etching the sacrificial layer and the device material layer.